This invention relates to a dynamic focusing circuit for use in conjunction with a cathode ray tube video circuit.
Most cathode ray tubes in use today include a generally rectangular screen on which is produced a picture or other information by means of a scanning electron beam. The electron beam, which is produced by an electron gun positioned behind the screen, sweeps horizontally across the screen a line at a time at a plurality of vertical levels, all under the control of vertical and horizontal deflection circuits. Because of the geometry of the cathode ray tube screen, the distance of travel of electrons from the electron gun to the screen is not uniform over the screen. That is, the travel distance increases as the beam moves toward the sides or the top and bottom of the screen. Of course, the travel distance would be greatest to the corners of the screen.
Also included in conventional cathode ray tube circuits is focus circuitry whose function is to "focus" or narrow the electron beam at the point it strikes the screen. Since the distance of travel of the electron beam to the screen varies with the position of the beam, the focusing requirements for the beam will likewise vary. Typically, cathode ray tube circuits are adapted to focus the electron beam at about the average travel distance from the electron gun to the screen so that the sharpness and clarity of the picture will be best at locations on the screen between the center and the sides and top and bottom.